CLEANUP: conn_stream: tree-wide rename to stconn (stream connector)

This renames the "struct conn_stream" to "struct stconn" and updates
the descriptions in all comments (and the rare help descriptions) to
"stream connector" or "connector". This touches a lot of files but
the change is minimal. The local variables were not even renamed, so
there's still a lot of "cs" everywhere.
diff --git a/src/activity.c b/src/activity.c
index 2638022..a84925d 100644
--- a/src/activity.c
+++ b/src/activity.c
@@ -621,7 +621,7 @@
 	unsigned long long tot_alloc_calls, tot_free_calls;
 	unsigned long long tot_alloc_bytes, tot_free_bytes;
 #endif
-	struct conn_stream *cs = appctx_cs(appctx);
+	struct stconn *cs = appctx_cs(appctx);
 	struct buffer *name_buffer = get_trash_chunk();
 	const char *str;
 	int max_lines;
@@ -849,7 +849,7 @@
 static int cli_io_handler_show_tasks(struct appctx *appctx)
 {
 	struct sched_activity tmp_activity[256] __attribute__((aligned(64)));
-	struct conn_stream *cs = appctx_cs(appctx);
+	struct stconn *cs = appctx_cs(appctx);
 	struct buffer *name_buffer = get_trash_chunk();
 	struct sched_activity *entry;
 	const struct tasklet *tl;